The present invention relates to implantable medical devices, and more particularly to an implantable cardioverter defibrillator (ICD) configured to provide a high efficiency defibrillation waveform.
An ICD continues to be a relatively large device for implantation in the human body. The size of the ICD is primarily determined by the battery and capacitors used therein. The size of the battery (or batteries, in some instances) and capacitors, in turn, is determined by the shock energy requirements for a defibrillation pulse. Thus, a design approach which reduces the energy requirements for defibrillation results in a direct reduction in the overall ICD size.
In existing ICD devices, the defibrillation waveform or pulse used to deliver a defibrillation shock to the heart is generated by first charging the equivalent of a single capacitor (most ICDs use two capacitors connected in series to function as a single capacitor, thereby reducing the working voltage requirements for each capacitor of the series stack, as explained below) to a desired charge level (voltage) and then discharging the single capacitor through the cardiac tissue for a prescribed period of time during a first or positive phase of the defibrillation waveform, and then reversing the polarity of the discharge for a second prescribed period of time during a second or negative phase of the defibrillation waveform, thereby producing a biphasic stimulation pulse or waveform. It should be noted that in this context the term xe2x80x9csingle capacitorxe2x80x9d is used to refer to a single capacitance, which may be, and usually is obtained by a hardwired connection of two capacitors in series such that the two series capacitors always function and act as though they were a single capacitor. (Two capacitors are connected in series in this manner in order to achieve a higher working voltage for the series-connected capacitor. That is, when two capacitors are connected in series, and each has a working voltage of, e.g., 375 volts (V), then the overall or total working voltage of the series combination becomes 750 V.)
The purpose of applying a defibrillation shock to the heart is to shock the heart out of a state of fibrillation, or other non-functional state, into a functional state where it may operate efficiently as a pump to pump blood through the body. To this end, the positive phase of the biphasic waveform is preferably a very high voltage that serves to synchronously capture as many heart membrane cells as possible. See, Kroll, xe2x80x9cA minimum model of the signal capacitor biphasic waveformxe2x80x9d Pace, November 1994. The negative phase of the biphasic waveform, in contrast, simply serves to remove the residual electrical charge from the membrane cells and bring the collective membrane voltage back to its original position or value. See, e.g., Kroll, supra; Walcott, et al., xe2x80x9cChoosing The Optimal Monophasic and Biphasic Wave-Forms for Ventricular Defibrillation,xe2x80x9d, Journal of Cardiovascular Electrophysiology (September 1995). A biphasic pulse generator of the type used in an ICD device is shown, e.g., in U.S. Pat. Nos. 4,850,357, issued to Bach, Jr.; and 5,083,562, issued to de Coriolis et al.
When a voltage shock is first applied to a membrane cell, the membrane does not respond to the shock immediately. Rather, the cell response lags behind the applied voltage. This time lag is more or less predictable in accordance with the Blair membrane model. See, e.g., Blair, xe2x80x9cOn the intensity-time relations for stimulation by electric currents. Ixe2x80x9d J. Gen Physiol., Vol. 15, pp. 709-729 (1932), and Blair, xe2x80x9cOn the intensity time relations for stimulation by electric currents. IIxe2x80x9d, J. Gen Physiol., Vol. 15, pp. 731-755 (1932); Pearce et al., xe2x80x9cMyocardial stimulation with ultrashort duration current pulses,xe2x80x9d PACE, Vol. 5, pp. 52-58 (1982). When the applied voltage comprises a biphasic pulse having a constant voltage level for the duration of the positive phase (a condition achievable only when the voltage originates from an ideal battery), the membrane cell response to the positive phase reaches a peak (i.e., is at an optimum level) at the trailing edge of the positive phase. Unfortunately, when the applied voltage originates from a charged capacitor, as is the case for an ICD device, the applied voltage waveform does not remain at a constant voltage level, but rather has a significant xe2x80x9ctiltxe2x80x9d or discharge slope associated therewith. Such tilt or slope causes the peak membrane cell response to occur at some point prior to the trailing edge of the positive phase, which is less than optimum. What is needed, therefore, is a way to optimize the applied voltage waveform so that a maximum membrane cell response occurs coincident with, or nearly coincident with, the trailing edge of the positive phase.
It is known in the art to switch the capacitors of an ICD from a parallel configuration during the positive phase of a biphasic defibrillation pulse to a series configuration during the negative phase of the biphasic defibrillation pulse. See, e.g., U.S. Pat. Nos. 5,199,429 (FIG. 7A) and 5,411,525. While such action produces a defibrillation waveform having a somewhat different shape, i.e., a waveform having a leading edge voltage of the second or negative phase which is approximately twice the trailing edge voltage of the first or positive phase, such action does little to achieve a maximum cell membrane response coincident with the trailing edge of the first or positive phase.
It is also known in the art to sequentially switch capacitors in an ICD device in order to allow waveform xe2x80x9ctailoringxe2x80x9d, e.g., prolong the positive phase duration by sequentially switching in a second charged capacitor as shown in FIG. 9 of U.S. Pat. No. 5,199,429, or by sequentially switching in second, third and fourth charged capacitors, as shown in FIG. 6C of U.S. Pat. No. 5,199,429. However, such xe2x80x9ctailoringxe2x80x9d still does not address the main concern of achieving a maximum cell membrane response coincident with the trailing edge of the positive phase.
It is thus evident that what is needed is a capacitor switching scheme and/or method for use within an ICD device which achieves a maximum cell membrane response near or coincident with the trailing edge of the positive phase.
It is also desirable to provide an ICD that is as small as possible. The limiting factor on ICD thickness is the diameter of the high-energy capacitors. As indicated above, current ICDs typically use two electrolytic capacitors. Current technology in electrolytic capacitors limits the stored voltage to about 370 V per capacitor. Therefore, the current approach is to use two large (xe2x89xa7180 xcexcF) capacitors to achieve the stored energy of xe2x89xa725J required for defibrillation. Therefore, the thickness of the ICD is determined by the diameter of the large (xe2x89xa7180 xcexcF) capacitors. There is thus a need for an ICD construction, which would permit the needed energy for defibrillation to be stored in the ICD, while allowing a thinner ICD thickness.
The present invention advantageously addresses the above and other needs.
The present invention generates a highly efficient first phase (which is usually a positive phase) of a biphasic defibrillation pulse by switching at least two charged capacitors, preferably three capacitors, from a parallel connection to a series connection during the first or positive phase of the defibrillation pulse. Such mid-stream parallel-to-series switch advantageously steps up the voltage applied to the cardiac tissue during the first phase. A stepped-up voltage during the first phase, in turn, gives an extra boost to, and thereby forces additional charge (current) into, the cardiac tissue cells, and thereby transfers more charge into the membrane of the excitable cardiac cell than would be transferred if the capacitors were continuously discharged in series.
Phase reversal, e.g., switching to a second or negative phase of the biphasic waveform) is timed to occur when the cell membrane voltage reaches its maximum value at the end of the first phase.
In accordance with one aspect of the invention, two capacitors are used within the ICD to produce a two-step waveform that outperforms the conventional one-step waveform. It will be shown that the two-step waveform requires a 15.6% lower leading edge, which may result in significantly less pain felt by the patient, and further translates into at 28.8% reduction in required stored energy. This reduction in leading edge amplitude and required stored energy is achieved by controlling the durations of the first and second steps in the two-step positive portion of the waveform.
In accordance with another aspect of the invention, three capacitors are used within the ICD in order to provide a thinner ICD. These three capacitors store the same energy as a two-capacitor ICD. These smaller capacitors have a smaller diameter and therefore the ICD can be made thinner.
Disadvantageously, using three capacitors instead of two creates its own set of problems that must be overcome by the present invention. Using three capacitors discharged in series results in: (a) high peak voltages (generally the peak voltage can be three times 370 V or 1110 V); and (b) a small discharge time constant, since the effective capacitance is that of a single capacitor divided by three (or 40 xcexcF if 120 xcexcF capacitors are used), resulting in a mismatch between the discharge (xcfx84=R*C, with R≈50xcexa9) and tissue (xcfx84m≈3 ms) time constants. Advantageously, the present invention addresses both of these concerns.
In accordance with another aspect of the invention, the capacitors of the ICD are reconfigured from a parallel configuration to a series configuration during the positive portion of the defibrillation pulse. While this concept may be used effectively with a two-capacitor ICD, it is preferred for purposes of the present invention that at least three capacitors be used, thereby allowing the ICD to be somewhat thinner that it otherwise could be.
It is therefore a feature of the present invention to provide an ICD that generates a highly efficient stimulation waveform that transfers more charge to the membrane of an excitable cardiac cell than has heretofore been possible using conventional, series-discharge configurations.
It is a further feature of the invention to provide an ICD design that results in a thinner ICD than has heretofore been possible using a conventional two-capacitor ICDs.